Nagasaki has called for a shift away from nuclear power as it marked the 66th anniversary of its atomic bombing at the end of World War II.
Mayor Tomihisa Taue said Japan must develop safer alternative energies such as solar, wind and biomass following the meltdowns at the Fukushima Daiichi nuclear plant in March.
''This March, we were astounded by the severity of the accident at the Fukushima Daiichi nuclear power station,'' Mr Taue said on Tuesday.
''As the people of a nation that has experienced nuclear devastation, we have continued the plea of 'No More Hibakusha!','' he said, using the Japanese word for the radiation victims.
''How has it happened that we are threatened once again by the fear of radiation? Have we lost our awe of nature? Have we become overconfident in the control we wield as human beings?''
Until the earthquake and tsunami on 11 March, Japan relied on nuclear power for about 30% of its energy needs and had planned to boost that to 50% by 2030.
Only 16 of the country's 54 reactors are currently operational, with most of the closed plants now undergoing safety checks.
More nuclear plants are due to go offline for regular checks and maintenance in coming months.
Mr Taue said that "no matter how long it will take, it is necessary to promote the development of renewable energies in place of nuclear power in a bid to transform ourselves into a society with a safer energy base."
His message echoed that of Hiroshima's mayor and of Prime Minister Naoto Kan, who has pushed for a significant reduction of nuclear energy in quake-prone Japan.
A record 44 countries sent representatives to the ceremony. For the first time, the United States sent an official to the Nagasaki event, after sending its ambassador to Japan last year to Hiroshima.
Nagasaki was devastated on 9 August, 1945, by a bomb nicknamed ''Fat Man'' which killed more than 70,000 people instantly or in the days and weeks that followed, from burns and radiation sickness.
Three days earlier, ''Little Boy,'' a four-tonne uranium bomb, was dropped on Hiroshima, killing an estimated 140,000 people.